1. Field of the Invention
The invention generally relates to a process for making a low density, rigid polyurethane foam. More particularly, the embodiment of the present invention relates to closed-cell, water blown, rigid polyurethane foams which may be processed with a low bulk density while providing enhanced mechanical performance and energy absorbing capability heretofore unachievable with urethane formulations that do not use toluene diisocyanate.
2. Prior Art
In many applications, polyurethane foams are used to absorb, or at least mitigate, the effects of high acceleration loads. In particular, low density polyurethane foams are widely used as a packing media while rigid, high density polyurethane foams are used in the construction of automobile bumpers and to stiffen door support frames. Additionally, many polyurethane foams currently using TDI (toluene diisocyanate) and MDI (diphenylmethane diisocyanate) prepolymers have not been intentionally optimized with respect to their toughness or impact properties. As such, there has been a growing need to develop polyurethane foam systems with improved energy absorbing capacity.
For example, U.S. Pat. No. 4,722,946 describes the production of energy attenuating viscoelastic polyurethane elastomers and foams, comprising mixtures of linear and branched polyol intermediates and polyisocyanates in the presence of a catalyst. Additionally, U.S. Pat. No. 4,696,954 describes the preparation of molded polyurethane foams characterized by high impact strength and good thermal stability. Furthermore, some limited success has been had with certain formulations using water as a blowing agent, and containing a polymer polyol (graft polyol) as necessary elements of the invention. U.S. Pat. Nos. 4,190,712, 4,212,954, and 4,116,893 disclose formulations for flexible or viscoelastic foams.
While many energy absorbing polyurethane foam compositions are known in the art, there is need for foam compositions that are simple and utilize commercially available materials producing light weight foams having superior mechanical properties.